Polyacrylamide has poor salt-resistance and must be combined with freshwater so as to reach economic and relatively stable viscosity for the polymer solution. Therefore, solving the problem of salt-resistance for the polyacrylamide turns into one of the hot topics of the research in the field of oilfield chemistry over the world.
A comb-shaped salt-resistant polymer thickening agent described in Chinese invention patent (CN 011367989) has been largely applied for class I oil reservoirs in oilfields. This thickening agent can be directly combined with sewage, and the thickening performance thereof is increased by more than 50% as compared to that of polyacrylamide. This thickening agent has significant effect of increasing oil production as well as decreasing water cut, and thus becomes the new generation of oil displacement agents used in the tertiary oil recovery for class I oil reservoirs, which has a range of permeability in favor of polymer flooding, in oilfields. However, such industrial product of comb-shaped salt-resistant polymer has a lager molecular weight and a slower dissolving rate than polyacrylamide, and thus it will encounter some troubles in injection performance when applied for class II oil reservoirs, which has a permeability range lower than that of in class I oil reservoirs, in oilfields. It is an emphasis for further increasing the enhanced oil recovery of oilfields that the tertiary oil recovery in class II oil reservoirs in oilfields will become more important. Because the permeability of class II oil reservoirs is lower than that of class I oil reservoir in oilfields, it is necessary to utilize a new salt-resistant polymer that has relatively low molecular weight, good salt-resistance, and dissolving rate similar to that of polyacrylamide.
With regards to the requirement of the tertiary oil recovery for class II oil reservoir in oilfields, the comb-shaped salt-resistant polymer thickening agent has been improved and a braided comb-shaped salt-resistant polymer thickening agent has also been successfully developed, which is applied in U.S. patent (U.S. Pat. No. 7,304,116) and Chinese invention patent (CN 2005100752911). This polymer has good salt-resistance with relatively low molecular weight, but, due to its lower dissolving rate in neutral water, the dissolving rate thereof could be similar to that of polyacrylamide only under certain base condition. Therefore, although the braided comb-shaped salt-resistant polymer thickening agent has good effect when used in combination flooding for class II oil reservoir in oilfields, it is still required to develop novel salt-resistant polymers used in polymer flooding for class II oil reservoir in oilfields.